Project summary/Abstract Osteomyelitis (OM) is a microbial-induced inflammatory bone disease that disrupts normal skeletal remodeling. Staphylococcus aureus is the principle etiologic agent, accounting for roughly 90% of all hematogenous cases in children. The treatment of OM frequently involves the use of intravenous antibiotics, but even with appropriate antimicrobial therapy, a subset of patients with OM will require surgical debridement. A majority of the research on OM to date has centered on adult OM, in particular related to orthopedic implants. Thus, there is an unmet clinical need based on gaps in our understanding of the pathogenesis of OM in the pediatric population. Herein, we propose to optimize mouse models of S. aureus-induced OM to reflect the pathophysiology of pediatric disease, paving the way for studies to elucidate underlying disease mechanisms and novel treatment approaches. This R21 falls into the category of model development ? currently there is no model for pediatric OM. Thus, the reward will be a new platform for the investigation of this understudied condition. Unlike in adults, OM in children typically occurs due to hematogenous spread of bacteria, but no current model features blood-born infection in growing animals. In this proposal, we will use a combination of intratibial and intravenous injection, following disease progression longitudinally with bioluminescence and in vivo microCT imaging. We will also perform dose responses, to find the lowest disease-causing inocula, which we expect will reflect about host-pathogen interactions better than high doses. The incidence of OM caused by methicillin-resistant S. aureus (MRSA) infections has increased, now reaching similar levels as methicillin sensitive (MSSA) strains. Effective antibiotics for MRSA strains are still available, but treatment periods are long and complications can be severe. There has been little research to understand if the higher morbidity of MRSA vs MSSA OM extends beyond antibiotic sensitivity to other aspects of disease initiation or progression. We will enable such work by establishing comparable models of OM with MRSA and MSSA strains from pediatric OM patients (ie. clinical isolates), in rapidly growing 6 week old mice. Bone infection in children is generally localized near the growth plate of long bones, a very active area for bone formation. We will investigate the hypothesis that the high bone anabolism found during growth in childhood contributes to development or progression of hematogenous OM. We will also determine whether the presence of a reparative state following mild bone injury in growing animals affects the course of hematogenous OM initiated shortly after the injury. Future projects will then focus on the cellular basis for these host effects on a range of bacterial strains, yielding insight into the pathogenesis of pediatric OM. Thus, our aims are as follows: Aim 1: Optimize and characterize direct tibial inoculation models of MSSA and MRSA- induced OM in growing mice. Aim 2: Develop models of hematogenous MSSA and MRSA OM in growing mice. Aim 3: Determine role of bone anabolism and repair in localization of hematogenous OM lesions.